Culbert Olson is one the most important men you probably never have heard of. He was the only Democrat to serve as governor of California between 1896 and 1958, and he lasted just one term—elected in 1938, and ousted in 1942. He was that rarest of birds among American politicians elected to high office, an atheist and free thinker.

He may be best known for refusing to say the words “so help me God,” substituting it with “I will affirm” …